The present invention reltaes generally to shaft position encoders, and in particular to smart shaft position encoders that contain dedicated logic and decoder circuits for converting raw sensor data into instrumentation-compatible format. The shaft position encoder apparatus of the present invention is especially useful for recording changes in liquid levels, such as river or reservoir levels, and indicating whether the change is up or down from the previous level. The system produces a processed electrical pulse signal for each discrete rise or fall in liquid level. As a result of signaling only increases or decreases exceeding a predetermined discrete amount, the shaft position encoder of the present invention has a low-duty cycle and normally draws only a small value of standby current. The particular implementation of the circuit obviates the need for a microprocessor and associated memory for storing liquid level signals.
Previously, it has been proposed in U.S. Pat. No. 4,313,114 of Lee et al., to provide a liquid level recording system including a shaft encoder pulse generator which transmits a group of electrical pulses for each measure change in shaft position or liquid level. The sequence of pulses in each group indicates whether the change is an increase or decrease from the previous reading. The system employs a microprocessor to measure the shaft position and direction of rotation and uses a temporary memory to store a difference signal corresponding to changes in the shaft position. This difference signal is periodically gated into a permanent memory if it exceeds a predetermined magnitude, or "deadband." Unlike the shaft position encoder of the present invention, this prior apparatus employs a periodically gated memory for storing the difference signal corresponding to the difference between successively measured liquid levels. It also provides slower tracking of shaft positon and consumes more power than the present invention. Further, this prior art recording apparatus is intolerant to wide swings in the power supply voltage. Lastly, the Lee et al. apparatus uses a microprocessor-based system external to the pulse generator to provide the logic and decoding functions necessary to interface with standard instrumentation equipment, rather than providing these functions internally.